1. Technical Field
The present invention relates to technologies for encrypting and accumulating data pieces having a size smaller than a unit size for encryption.
2. Background Art
In recent years, tags having various sensor functions as well as a communication function have been developed, and various kinds of application software using such tags have been designed accordingly. The sensor functions include, for example, the function of a temperature sensor. As a possible application, a tag having a temperature sensor may be attached to an object such as a perishable product and an art work to measure the temperature of the object during the transportation thereof. Another possible application is to use such a tag to provide a healthcare system for patients who require regular or long-term measurement of their body temperatures. In this way, a tag having such a measurement function (hereinafter called “a sensor tag”) attached to an object such as a product being transported and a patient's body is capable of obtaining the state of the object regularly. The state data obtained through the measurement is accumulated temporarily in the sensor tag. After the measurement for a given period, the sensor tag is removed from the product or the patient's body, and the accumulated state data is read out by using the communication function. Microminiaturized tags are capable of conducing regular measurement without making transporters or patients conscious of the presence of the sensor tags.
In the case of using sensor tags to provide a healthcare system for patients, it is necessary to guarantee the security of the data accumulated in each sensor tag. This is because the biological data obtained through the measurement is privacy for each patient. Thus, the biological data should be prevented from leaking, in case a sensor tag attached to the patient's body falls off without being noticed by the patient and is picked up by a third party, or the sensor tag falls into the hands of a third party because of careless handling of the tag after the biological data has been read out from it. In view of such risks, in order to prevent the biological data obtained through the measurement from leaking, it is necessary to provide a mechanism for encrypting the data within the sensor tag and accumulating it in encrypted form.
There are two major methods for encryption. One is a method with use of secret key cryptography, and the other is a method with use of public key cryptography. Secret key cryptography is characterized by that it uses an encryption key and a decryption key which are identical. Public key cryptography is characterized by that it uses an encryption key (i.e. public key) and a decryption key (i.e. private key) which are different. In both cases, only the encryption key is to be stored in the sensor tag. Here, suppose the case where the sensor tag is analyzed and the data inside leaks. In this case, it means that the encryption key can be analyzed.
In the case of secret key cryptography, the encrypted data in the sensor tag can be easily decrypted by using the encryption key, because the encryption key is identical to the secret key. Hence, it can not guarantee a high security level.
In the case of public key cryptography on the other hand, it is almost impossible to derive the decryption key from the encryption key, even though the encryption key is obtained. Hence, it is impossible to decrypt the encrypted data in the sensor tag. In this way, public key cryptography used for encryption of the accumulate data achieves a high security level. There are various kinds of public key cryptography, including RSA (Rivest Shamir Adleman) encryption, ElGamal encryption, elliptic curve ElGamal encryption, and NTRU encryption.